Rational design and synthesis of NixCo3-xO4 nanoparticles derived from multivariate MOF-74 for supercapacitors

Mixed transition metal oxides have attracted much attention recently due to their potential application in energy and environmental sciences. The emergence of multivariate MOFs recently has attracted great research attention and provides an opportunity for multivariate mixed-metal oxides. In this work, five isostructural, single-phase MOF-74 structures with different divalent metals (MOF-74-Co, MOF-74-Ni, MOF-74-NiCo1, MOF-74-NiCo2 and MOF-74-NiCo4) were synthesised by varying the mole ratios of Ni/Co mixed-metal ions. After annealing at the appropriate temperature, Co3O4, NiO and three kinds of NixCo3-xO4 mixed-metal oxide nanoparticles with high surface area were easily obtained and the metal ratio was readily controlled, enabling us to systematically investigate the effect of different metal species amounts on the electrochemical properties of the mixed-metal oxide materials. When these metal oxides were used as electrode materials for supercapacitors, we found that the mixed-metal oxides NixCo3-xO4 obtained from bimetallic MOF-74-NiCo structures had obvious advantages compared with the monometallic oxides of MOF-74-Ni and MOF-74-Co. In particular, the NixCo3-xO4-1 with the Ni/Co metal ratio 1 : 1 exhibited the highest capacitance of 797 F g(-1) and excellent cycling stability.